In some internal combustion engines disposed in vehicles and the like, there is provided an evaporation purge system in order to prevent evaporative fuel, which fuel is generated in a fuel tank, etc., from leaking into the ambient air from the fuel tank. In such an evaporation purge system, the evaporated fuel is introduced into a canister from the fuel tank through an evaporation passage. The canister contains an absorbent material such as activated carbon. Such introduced fuel is at first absorbed and retained in the canister, but is liberated and discharged from the canister when the engine is run, and is then purged into an intake passage through a purge passage for combustion in the engine.
In the evaporation purge system, when abnormalities such as faults or breakage occur in the canister, the passages, or valves, etc., then it follows that the evaporative fuel leaks into the atmosphere. In order to avoid this, some of the evaporation purge systems include an abnormality-diagnosing device. Examples of such abnormality-diagnosing devices are disclosed in published Laid-Open Japanese Patent Application Nos. 5-125997, 5-180099, 5-180100, 5-180101, 5-187332, 5-223019, and 4-136468.
In the abnormality-diagnosing device disclosed in No. 5-125997, when a vehicle stops, a purge control valve of a canister as well as a block valve for blocking an atmosphere aperture of the canister are completely closed during engine idle operation, thereby causing a space between a fuel tank and an intake passage to be enclosed in a gas tight condition under atmospheric pressure. In this state, the pressure is measured and how much the pressure varies is determined. Next, the purge control valve is fully opened to introduce negative intake pipe pressure, and then it is measured how much the pressure varies under such negative pressure, gas tight condition. When the latter pressure variation amount under the negative pressure is greater than the former pressure variation amount under the atmospheric pressure, then the abnormality-diagnosing device detects the presence of leakage.
In the abnormality-diagnosing device disclosed in No. 5-180099, a purge-side valve and a canister atmosphere aperture valve are both closed, and then the internal tank pressure achieves a predetermined pressure, whereby the pressure inside the system reaches a near-stable value. Such a pressure value and another value, which is measured after lapse of a predetermined period of time, are used to calculate a variation rate. When the variation rate is greater than a threshold value, it means that there is leakage in excess of a prescribed level of leakage. Then, the abnormality-diagnosing device judges the system as a failure, and turns on a warning lamp.
In the abnormality-diagnosing device disclosed in No. 5-180100, a variation rate is calculated on the basis of: respective internal tank pressures which are measured when the canister atmosphere aperture valve and the purge-side valve are each closed; and, another internal tank pressure after elapse of a given time period. The calculated variation rate is then compared in magnitude with a threshold value, thereby causing the device to diagnose possible system failures. Thereafter, the canister atmosphere aperture valve is opened, but the purge-side valve is prohibited from being opened in order to retain such an isolated state until the internal tank pressure achieves the atmospheric pressure or positive pressure. In this way, the evaporative fuel inside the canister is purged back into the fuel tank.
In the abnormality-diagnosing device disclosed in No. 5-180101, when the internal tank pressure achieves a value, which is greater toward negative pressure than a predetermined leakage determination-starting negative pressure, after the purge-side valve and the canister atmosphere aperture valve are both closed, then the purge-side valve is judged as a failure in valve opening. Then, the canister atmosphere aperture valve is opened, and the warning lamp is switched on. When the internal tank pressure does not achieve a predetermined value, even with lapse of a predetermine period of time, which predetermined value lies somewhat toward negative pressure with reference to the atmospheric pressure, then the canister atmosphere aperture valve is judged as a failure in valve closing, and the warning lamp is switched on.
In the abnormality-diagnosing device disclosed in No. 5-187332, an opening in a rotational speed control valve is adjusted so as to achieve a target revolving speed during engine idle operation, thereby controlling intake air quantity. An air-fuel ratio is controlled at a given level by means of an oxygen sensor. In such a situation, a purge control valve is forcedly switched into a completely closed state and a predetermined opening state, and then it is determined at this time how much the opening degree of the aforesaid rotational speed control valve is varied. When such a variation amount of the opening of the rotational speed control valve falls out of a predetermined permissive range, then the abnormality-diagnosing device determines the occurrence of abnormalities, and then warns the user thereof by turning on the warning lamp.
In the abnormality-diagnosing device disclosed in No. 5-223019, a variation in pressure inside the system for a predetermined period of time is calculated after the purge-side valve is opened, but the canister atmosphere aperture valve is closed. When such a variation rate is less than a predetermined value, it means the presence of a large leakage, thereby making a failure determination. When the variation rate is determined to be greater than the predetermined value, then the purge-side valve and the canister atmosphere aperture valve are both closed after an internal negative tank pressure reaches a predetermined value, whereby a failure diagnosis is made on the basis of how much the pressure inside the system is varied for a predetermined period of time.
In the abnormality-diagnosing device of No. 4-136468, an evaporative fuel processor includes means for absorbing fuel vapor in the fuel tank and a purge system for supplying the fuel vapor to an intake system of an engine. A failure diagnosis of the purge system is made on the basis of detected variations in an air-fuel ratio in the engine. The evaporative fuel processor comprises a failure-determining condition change means for changing a failure-determining condition, depending upon a rise in a fuel vapor pressure inside the fuel tank, so as to enhance the accuracy of failure determination. The aforesaid failure-determining condition is determined in accordance with operated states of both the purge system and the intake system.
In an abnormality-diagnosing device for diagnosing abnormalities in the evaporation purge system, an atmosphere valve for opening and closing the canister to/from the atmosphere is closed, while a purge valve is opened. In this way, negative intake pressure is established until internal tank pressure "PTNK" of the fuel tank reaches a target internal tank pressure "POTNK", as illustrated in FIG. 3. Then, the abnormality-diagnosing device diagnoses the evaporation purge system as abnormal when "PTNK" does not achieve "POTNK" after lapse of given time "t3" from the moment when the atmosphere valve is closed, but the purge valve is opened. In addition, when "PTNK" reaches "POTNK", then the abnormality-diagnosing device closes the purge valve, thereby causing the evaporation passage system to remain closed. Then, the abnormality-diagnosing device diagnoses the evaporation purge system as abnormal when internal tank pressure variation "DPTNK" is greater than internal tank pressure variation-determining value "PLEAK" after lapse of given time "t1" from the moment when the purge valve was closed.
However, there is a problem with the evaporation purge system which is designed to regulate a purge flow rate by the purge valve being driven based on a duty value. As illustrated in FIG. 4, the problem is that the purge flow rate decreases with a decrease in atmospheric pressure as the altitude rises from plains (i.e. low altitudes) to uplands (i.e. high altitudes), even with the duty value for the driving of the purge valve being invariable.
As a result, when the abnormality-diagnosing device executes an abnormality diagnosis at high altitudes, "PTNK" is slower in reaching "POTNK", even after the negative intake pressure is established by the atmosphere valve being closed, but the purge valve being opened; and, in some cases, "PTNK" does not reach "POTNK", even when given time "t3" has elapsed. This causes an inconvenience in that the abnormality-diagnosing device erroneously judges the evaporation purge system as abnormal.
In addition, since fuel in a fuel tank is more readily subject to vaporization with an increase in altitude, then given time "t3" is made longer. As a result, "PTNK" does note achieve "POTNK", even with lapse of time "t3". This causes another inconvenience in that the abnormality-diagnosing device misdiagnoses the evaporation purge system as abnormal.
Another inconvenience is that, when the evaporation purge system purges the evaporative fuel after "PTNK" achieved "POTNK" during idle operation and reduced load operation of the engine and, particularly, in a state in which the canister absorbs a large quantity of evaporative fuel, or in which the evaporative fuel readily occurs in the fuel tank because of an elevated altitude or temperature, then an air/fuel ratio is made richer, with a consequential failure in drivability or aggravation of harmful exhaust component values.
Further, there is a similar inconvenience when the purge valve is opened rapidly for purging of the evaporative fuel in a state in which the canister absorbs a large quantity of evaporative fuel, or when the evaporative fuel readily occurs in the fuel tank because of an elevated altitude or temperature, then the result is a richer air/fuel ratio with malfunctioned drivability and/or increased harmful exhaust component values.